Solid state membranes formed from oxygen ion conducting materials are beginning to show promise for use in commercial processes for separating oxygen from oxygen-containing streams. Envisioned applications range from small scale oxygen pumps for medical use to large gas generation and purification plants. This technology encompasses two distinctly different membrane materials, solid electrolytes and mixed conductors. Membranes formed from mixed conductors are preferred over solid electrolytes in processes for separating oxygen from oxygen-containing feeds because mixed conductors conduct both oxygen ions and electrons, and can be operated without external circuitry such as electrodes, interconnects and power-supplies. In contrast, solid electrolytes conduct only oxygen ions, and external circuitry is needed to maintain the flow of electrons to maintain the membrane ionization/deionization process. Such circuitry can add to unit cost, as well as complicate cell geometry.
Membranes formed from solid electrolytes and mixed conducting oxides are oxygen selective and can transport oxygen ions through dynamically formed oxygen anion vacancies in the solid lattice when operated at temperatures typically above about 500.degree. C. Examples of solid electrolytes include yttria-stabilized zirconia (YSZ) and bismuth oxide. Examples of mixed conductors include titania-doped YSZ, praseodymia-modified YSZ, and, more importantly, various mixed metal oxides some of which possess the perovskite structure. Japanese Patent Application No. 61-21717 discloses membranes formed from multicomponent metallic oxides having the perovskite structure represented by the formula La.sub.1-x Sr.sub.x Co.sub.1-y Fe.sub.y O.sub.3-d wherein x ranges from 0.1 to 1.0, y ranges from 0.05 to 1.0 and d ranges from 0.5 to 0.
Membranes formed from mixed conducting oxides which are operated at elevated temperatures can be used to selectively separate oxygen from an oxygen-containing feed when a difference in oxygen partial pressures exist on opposite sides of the membrane. Oxygen transport occurs as molecular oxygen is dissociated into oxygen ions, which ions migrate to the low oxygen partial pressure side of the membrane where the ions recombine to form oxygen molecules, and electrons migrate through the membrane in a direction opposite the oxygen ions to conserve charge.
The rate at which oxygen permeates through a membrane is mainly controlled by three factors. They are (a) the kinetic rate of the feed side interfacial oxygen exchange, i.e., the rate at which oxygen molecules in the feed are converted to mobile oxygen ions at the surface of the feed side of the membrane; (b) the diffusion rates of oxygen ions and electrons within the membrane; and (c) the kinetic rate of the permeate side interfacial oxygen exchange, i.e., the rate at which oxygen ions in the membrane are converted back to oxygen molecules and released on the permeate side of the membrane.
U.S. Pat. No. 5,240,480 to Thorogood, et al, incorporated herein by reference, addressed the kinetic rate of the feed side interfacial oxygen exchange by controlling the pore size of the porous structure supporting a non-porous dense layer. Numerous references, such as U.S. Pat. No. 4,330,633 to Yoshisato et al, Japanese Kokai No. 56[1981]-92,103 to Yamaji, et al, and the article by Teraoka and coworkers, Chem. Letters, The Chem. Soc. of Japan, pp. 503-506 (1988) describe materials with enhanced ionic and electronic conductive properties.
Typical of metal oxide membrane references is Japanese Patent Application 61-21717, described above. When an oxygen-containing gaseous mixture at a high oxygen partial pressure is applied to one side of a membrane having a dense layer formed from the enumerated oxide, oxygen will adsorb and dissociate on the membrane surface, become ionized and diffuse through the solid and deionize, associate and desorb as an oxygen gas stream at a lower oxygen partial pressure on the other side of the membrane.
The necessary circuit of electrons to supply this ionization/deionization process is maintained internally in the oxide via its electronic conductivity. This type of separation process is described as particularly suitable for separating oxygen from a gas stream containing a relatively high partial pressure of oxygen, i.e., greater than or equal to 0.2 atm. Multicomponent metallic oxides which demonstrate both oxygen ionic conductivity and electronic conductivity typically demonstrate an oxygen ionic conductivity ranging from 0.01 ohm.sup.-1 cm.sup.-1 to 100 ohm.sup.-1 cm.sup.-1 and an electronic conductivity ranging from about 1 ohm.sup.-1 cm.sup.-1 to 100 ohm.sup.-1 cm.sup.-1.
Some multicomponent metallic oxides are primarily or solely oxygen ionic conductors at elevated temperatures. An example is (Y.sub.2 O.sub.3).sub.0.1 (Zr.sub.2 O.sub.3).sub.0.9 which has an oxygen ionic conductivity of 10 ohm.sup.-1 cm.sup.-1 at 1000.degree. C. and an ionic transport number (the ratio of the ionic conductivity to the total conductivity) close to 1. European Patent Application EP 0399833A1 describes a membrane formed from a composite of this oxide with a separate electronically conducting phase, such as platinum or another noble metal. The electronic conducting phase will provide the return supply of electrons through the structure allowing oxygen to be ionically conducted through the composite membrane under a partial pressure gradient driving force.
Another category of multicomponent metallic oxides exhibit primarily or solely electronic conductivity at elevated temperatures and their ionic transport numbers are close to zero. An example is Pr.sub.x In.sub.y O.sub.z which is described in European Patent Application EP 0,399,833 A1. Such materials may be used in a composite membrane with a separate oxygen ionic conducting phase such as a stabilized ZrO.sub.2. A membrane constructed from a composite of this type may also be used to separate oxygen from an oxygen-containing stream, such as air, by applying an oxygen partial pressure gradient as the driving force. Typically, the multicomponent oxide electronic conductor is placed in intimate contact with an oxygen ionic conductor.
Organic polymeric membranes may also be used for oxygen separation. However, membranes formed from mixed conducting oxides offer substantially superior oxygen selectivity than polymeric membranes. The value of such improved selectivity must be weighed against the higher costs associated with building and operating plants employing membranes formed from mixed conducting oxides which plants require heat exchangers, high temperature seals and other costly equipment. Typical prior art membranes formed from mixed conducting oxides do not exhibit sufficient oxygen permeance (defined as a ratio of permeability to thickness) to justify their use in commercial oxygen separation applications.
Oxygen permeance through solid state membranes is known to increase proportionally with decreasing membrane thickness, and mechanically stable, relatively thin membrane structures have been widely studied.
A second article by Teraoka et al, Jour. Ceram. Soc. Japan. International Ed, Vol 97, pp 458-462, (1989) and J. Ceram. Soc. Japan, International Ed, Vol 97, pp 523-529, (1989), for example, describes solid state gas separation membranes formed by depositing a dense, nonporous mixed conducting oxide layer, referred to as "the dense layer", onto a porous mixed conducting support. The relatively thick porous mixed conducting support provides mechanical stability for the thin, relatively fragile dense, nonporous mixed conducting layer. Structural failures due to thermo-mechanical stresses experienced by the membranes during fabrication and use were substantially minimized due to the chemical compatibility of the respective membrane layers. Based upon considerations limited to dense layer thickness, Teraoka and coworkers expected the oxygen flux to increase by a factor of 10 for a membrane having a mixed conducting porous layer and a thin mixed conducting dense layer compared to a standard single layered dense, sintered mixed conducting disc. However, they obtained an increase of less than a factor of two.
Researchers are continuing their search for solid state conductive membranes which exhibit superior oxygen flux without sacrificing mechanical and physical compatibility of the composite membrane.